This means sharpness of vision or the visual ability to resolve fine detail (usually measured by a Snellen chart). Visual acuity is expressed as a fraction. The top number refers to the distance you stand from the chart. This is usually 20 feet. The bottom number indicates the distance at which a person with normal eyesight could read the same line you correctly read. For example, 20/20 is considered normal. 20/40 indicates that the line you correctly read letters at 20 feet that could be read by a person with normal vision at 40 feet. The test is performed when there are problems or changes in vision.